Magnetoresistive Random Access Memory (MRAM) is a non-volatile memory technology that uses magnetic elements. For example, Spin Transfer Torque Magnetoresistive Random Access Memory (STT-MRAM) uses electrons that become spin-polarized as the electrons pass through a thin film (spin filter). STT-MRAM is also known as Spin Transfer Torque RAM (STT-RAM), Spin Torque Transfer Magnetization Switching RAM (Spin-RAM), and Spin Momentum Transfer (SMT-RAM).
FIG. 1 illustrates a conventional STT-MRAM bit cell 100. The STT-MRAM bit cell 100 includes magnetic tunnel junction (MTJ) storage element 105, a transistor 101, a bit line 102 and a word line 103. The MTJ storage element is formed, for example, from at least two ferromagnetic layers (a pinned layer and a free layer), each of which can hold a magnetic field or polarization, separated by a thin non-magnetic insulating layer (tunneling barrier). Electrons from the two ferromagnetic layers can penetrate through the tunneling barrier due to a tunneling effect under a bias voltage applied to the ferromagnetic layers. The spin polarized electrons tunneling through to the free layer may transfer their torque or angular momentum to the magnetic elements of the free layer, thus affecting the magnetic polarization of the free layer.
The magnetic polarization of the free layer can be reversed so that the polarity of the pinned layer and the free layer are either substantially aligned (parallel) or opposite (anti-parallel). The resistance of the electrical path through the MTJ will vary depending on the alignment of the polarizations of the pinned and free layers. This variance in resistance can be used to program and read the bit cell 100. The STT-MRAM bit cell 100 also includes a source line 104, a sense amplifier 108, read/write circuitry 106 and a bit line reference 107.
For example, the bit cell 100 may be programmed such that a binary value “1” is associated with an operational state wherein the polarity of the free layer is parallel to the polarity of the pinned layer. Correspondingly, a binary value “0” may be associated with an anti-parallel orientation between the two ferromagnetic layers. A binary value may thus be written to the bit cell by changing the polarization of the free layer. A sufficient current density (typically measured in Amperes/centimeter2) generated by the electrons flowing across the tunneling barrier is required to change the polarization of the free layer. The current density required to switch the polarization of the free layer is also called switching current density. Decreasing the value of the switching current density leads to beneficially lowering the power consumption of the MTJ cells. Additionally, lower switching current density enables smaller device dimensions and a correspondingly higher density of MTJ cells in an STT-MRAM integrated circuit.
The switching current density is dependent on the ability of electrons flowing across the tunneling barrier to efficiently transfer their spin torque to the magnetic elements of the free layer. Introducing a non-uniformity in the electrical current path created by the flow of electrons can advantageously lead to a more efficient transfer of the spin torque, thereby leading to a more efficient switching behavior and lower switching current density. However, conventional MTJ architectures promote a uniform current path across the MTJ bit cell. Accordingly, there is a need for architectures which can promote a non-uniform current path across the MTJ bit cells.